Preliminary Assessment of the Dose near the Shield Plug at Fukushima Daiichi Unit 2 (1F2)

Since the accident at Fukushima Daiichi on March 11, 2011, caused by the Great East Japan earthquake and subsequent tsunami, there have been significant coordinated international efforts to better understand the accident progression, phenomenology and consequences. Of particular note is the Organisation for Economic Cooperation and Development Nuclear Energy Agency's (OECD/NEA) Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF Project). Phase 1 of this ongoing effort focused on modeling the modeling of the three beyond design basis accidents at the site for the first six days of the accident. Phase 2 of the effort is a forensic effort and focuses on extending the accident simulations out to 21 days and assessing the consequences of the accident. This effort aims to inform the government of Japan and The Tokyo Electric Power Company (TEPCO)'s ongoing effort to decommission and decontaminate the site. One of the key questions posed by TEPCO in its efforts to decommission the site is the location of released radionuclides (RNs) that result in anomalously high dose rate measurements on and near the shield plugs of all three units. In this summary, we provide insight into the possible location of the source resulting in this high dose. Previous MELCOR accident progression analyses have predicted leakage of the containment through the drywell head, directly below the shield plug. Using this plausible accident scenario, MELCOR is able to predict the radionuclide source to the region below the shield plug and possible deposition amounts within the gaps of the shield plug. Using this predicted source and the transport and shielding code MCNP6 the dose in and around the shield plug of 1F2 was assessed. Unit 2 was chosen since the refueling bay is still intact and therefore a simpler initial problem compared to Units 1 and 3. (authors)